This paper presents an atomization mechanism of a spray injected into the low-pressure field, as the subject of injection system in a suction manifold of gasoline engine. Pure liquid fuel, which is n-Pentane or n-Hexane is injected into quiescent gaseous atmosphere at room-temperature and low- pressure through pintle type electronic control injector. Fuel sprays are observed by taking photographs for variation of the back pressure and the changes in spray characteristics with the back pressure below atmospheric pressure are examined in detail. In particular, in the case of the back pressure below the saturated vapor pressure of fuel, the atomization mechanism is discussed from a viewpoint of flash boiling phenomena, those are bubble growth rate and so on. The results show that the saturated vapor pressure of fuel is the most significant factor and the spray characteristics can be arranged with the pressure difference between the back pressure and the saturated vapor pressure for various fuels, corresponding to the intensity of the flash boiling. Then, in the case of the back pressure below the saturated vapor pressure, the atomization process can be explained in terms of the nucleation process and the growth rate of cavitation vapor bubbles due to the flash boiling, corresponding to the pressure difference between the back pressure and the saturated back vapor pressure. Further a certain model describing the fuel vapor formation rate is proposed considering the distribution of initial bubble nuclei in the fuel and the bubble growth process, based upon the cavitation bubble analysis in the flash boiling phenomena.* Number in bracket designate references at end of paper.RECENTLY, A MULTI-POINT INJECTION SYSTEM has become to be applied in gasoline engines predominantly owing to its high response and control quality of fuel supply. In this system, liquid fuel is injected into the relative low-pressure atmosphere through an electronic control injector. And in some operating conditions of engine, a great deal of unburnt hydrocarbon is emitted with exhaust gas because of its low quality of the spray atomization. Therefore, it is important to reveal the atomization process of the spray injected into the low-pressure field and especially to examine the changes in spray characteristics with the surrounding back pressure with reference to performance and exhaust emissions of the engine.In general, injected fuel from the pintle type injector appears in the form of fuel film with conical shape near the injector and the breakup of fuel film into droplets is not completed immediately. In fact, aerodynamic forces and the instability of fuel film due to the turbulence provided at the injector exit and the resistance by the atmospheric density cause unstable wave growth leading to the disruption of the film flow resulting to liquid ligaments and to droplets. However, when liquid fuel is injected into the back pressure below the equilibrium pressure, so-called the saturated vapor pressure, for the liquid temperature, the liquid spray disintegrates into fine droplets by evolution of the vapor owing to flash boiling. The flash boiling phenomena, that is vapor bubble growth by cavitation, occur when a liquid, initially in a subcooled state, is rapidly depressed to a pressure below the saturated vapor pressure to initiate a rapid boiling process.In connection with the flash boiling fuel injection, superheated liquid jets have often been researched as the subject of the study. For instance, Brown & York [1]* have described the relation between the droplet mean size and the growth rate constant of the bubbles, and Lienhard & James [2] have presented the breakup length of the flashing superheated jets. And then, Sudo et al. [3] have measured the droplet size distribution in Methanol flash boiling spray. Further, the effects of the nuclei number and the idle time of bubble growth rate on the atomization process have been reported by Suzuki et al. [4], and research on analytical bubble growth rate for practical injectors has been conducted by Oza & Sinnamon [5].This study is intended for investigating the atomization mechanism of the spray injected into room-temperature and low-pressure field through a pintle type electric control injector. In particular, the change in spray characteristics with the back pressure below the atmospheric pressure are examined in detail. And, when the back pressure is below the saturated vapor pressure of fuel, the spray characteristics are discussed from a viewpoint of flash boiling, consisting the nucleation process and the bubble growth rate. In addition, a model which describes the fuel vapor formation rate is proposed by considering the distribution of initial bubble nuclei in the fuel and the bubble growth process, based upon the cavitation bubble analysis in the flash boiling phenomena. Experiments were performed using two fuels, n-Pentane and n-Hexane.